feat: Integrate shadcn/ui design system

- Install shadcn/ui dependencies (CVA, clsx, tailwind-merge, lucide-react)
- Install Radix UI primitives (slot, label)
- Create utility helper (lib/utils.ts with cn function)
- Update Tailwind config for shadcn/ui theme support
- Add CSS variables for theming in globals.css (Tailwind v4 syntax)
- Add shadcn/ui components: Button, Card, Input, Label
- Update JourneyForm with shadcn/ui components and icons
- Update DataPreview with shadcn/ui components and icons
- Improve UI consistency and accessibility

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

frontend/tailwind.config.js

@@ -6,7 +6,48 @@ module.exports = {
     './app/**/*.{js,ts,jsx,tsx,mdx}',
   ],
   theme: {
-    extend: {},
+    extend: {
+      colors: {
+        border: "hsl(var(--color-border))",
+        input: "hsl(var(--color-input))",
+        ring: "hsl(var(--color-ring))",
+        background: "hsl(var(--color-background))",
+        foreground: "hsl(var(--color-foreground))",
+        primary: {
+          DEFAULT: "hsl(var(--color-primary))",
+          foreground: "hsl(var(--color-primary-foreground))",
+        },
+        secondary: {
+          DEFAULT: "hsl(var(--color-secondary))",
+          foreground: "hsl(var(--color-secondary-foreground))",
+        },
+        destructive: {
+          DEFAULT: "hsl(var(--color-destructive))",
+          foreground: "hsl(var(--color-destructive-foreground))",
+        },
+        muted: {
+          DEFAULT: "hsl(var(--color-muted))",
+          foreground: "hsl(var(--color-muted-foreground))",
+        },
+        accent: {
+          DEFAULT: "hsl(var(--color-accent))",
+          foreground: "hsl(var(--color-accent-foreground))",
+        },
+        popover: {
+          DEFAULT: "hsl(var(--color-popover))",
+          foreground: "hsl(var(--color-popover-foreground))",
+        },
+        card: {
+          DEFAULT: "hsl(var(--color-card))",
+          foreground: "hsl(var(--color-card-foreground))",
+        },
+      },
+      borderRadius: {
+        lg: "var(--radius)",
+        md: "calc(var(--radius) - 2px)",
+        sm: "calc(var(--radius) - 4px)",
+      },
+    },
   },
   plugins: [],
 }